(FORTUNE Magazine) – If you want a peek at the Next Big Thing in computing, check out Web calendars. Many Internet portals now offer online calendars that keep track of your appointments, reminders, and to-do lists. Like Web mail, which I reviewed in my last column (Oct. 11, in the fortune.com archive), these online calendars make sense if you're constantly on the go; they let you check your schedule regularly from any computer that has a Web connection. That's great. But as with many Web applications, getting access to an online calendar can take forever, especially when the network is overloaded. That's not so great.

With these pluses and minuses, the calendars offer a preview of the potential benefits and problems of online applications, a hot new category of software that will be delivered by companies calling themselves applications service providers. ASPs promise to fundamentally alter the relationship between user and software by offering complex applications over the Internet. Instead of buying software, you'll just log in to a Website whenever you need an application. That doesn't tax your hard drive, and it gives corporate systems managers more control over what's on users' desktops. But as you can imagine, this new paradigm doesn't sit too well with companies that have built empires on shrink-wrapped products (for more on Microsoft and the new world of computing, see "The $100 Billion Friendship").

ASPs are the future; Web calendaring is here. Until recently, keeping track of schedules on your PC required shrink-wrapped software like Outlook, Act, or Lotus Organizer. Free online calendars do the same thing and more. The "more" is what's most interesting--and most difficult for the shrink-wrapped products to match.

Consider Netscape's calendar (available to anyone who wants to sign up at www.netscape.com/calendar). It's a part of an online suite called Netscape Contact, which includes WebMail, an address book, and instant messaging. The calendar tracks your schedule, with appointments listed in a grid on your browser--you can choose a daily, weekly, or monthly view. You can even set alarms that send you e-mail reminders.

But the real fun here is Netscape's "public events" function, which taps into a vast database of public events. While several Web calendars offer this feature, Netscape's version was the most fine-grained. I had a choice of ten categories, from Internet events to concerts to professional sports, and I could even drill down to a specific event or location. How does it work? Well, when I selected cultural events in New York City, I was shown the listings of dozens of venues, from Radio City Music Hall to the performing arts center at Queens College. When I checked off a location, any events there were added to my calendar. I could have also used this to track a local hockey team, reruns of a favorite TV show, or CD releases by a favorite artist. It's a neat idea: no more forgetting a concert or play that you had meant to attend. Well, that's the theory, at least. In practice, the database, provided by a company called When.com, was missing several events that I was interested in.

Other online calendars, including Yahoo, Day-Timer Digital (www.digital.daytimer.com), and Portico, a voice-controlled messaging and e-mail service (www.portico.com), offer another cool feature: They let you synchronize your calendar with desktop calendars or a hand-held computer. By downloading software from their sites, PC users (sorry, no Mac versions so far) can move data from the Web to a hand-held and vice versa.

This process is still somewhat buggy. After several frustrating efforts to upload the calendar on my 3Com Palm to Portico, I gave up and called tech support. A helpful technician told me I couldn't synchronize through a corporate firewall--which seems a bit of a problem, given that business users are Portico's target market. But even when I connected through a modem, I still couldn't get my appointments into Portico. My dream of convergence heaven--an efficient digital assistant reading my appointments and messages to me over the phone--will have to wait.

I had better luck with Yahoo and Day-Timer. Using a Cassiopeia E-100, a hand-held device running Windows CE, I could easily pump several months of calendar items into both services. Having finally put my calendar on the Internet, I could share it with an assistant who uses the Mac. Which is another reason there may be sleepless suits in Seattle.